Glasses and glass-ceramics and products made therefrom

ABSTRACT

Thermally crystallizable glasses and glass-ceramics having a narrow critical range of compositions coming within the SiO 2  -Al 2  O 3  -Li 2  O and SiO 2  -Al 2  O 3  -Li 2  O-ZnO systems and glass-ceramic products made therefrom including heat exchangers, such as regenerators and recuperators used with gas turbine engines, thermal reactors and catalytic reactors for the gaseous exhausts of combustion engines, and other products which are dimensionally stable at temperatures of at least 1500°F. and above, have very low coefficients of thermal expansion, and have a modulus of rupture of at least about 10,000 psi which is retained when operating at temperatures of at least 1500°F., over an extended period of time.

This is a division of application Ser. No. 229,959, now U.S. Pat. No.3,841,950, filed Feb. 28, 1972 which is relied upon and the entiredisclosure and specification of which is hereby incorporated byreference. Which in turn is a continuation-in-part of copending U.S.Pat. application Ser. No. 146,664, filed May 25, 1971 now abandoned.

In U.S. Pat. application Ser. No. 30,859 filed Apr. 22, 1970, and inU.S. Pat. application Ser. No. 146,665 filed May 25, 1971, both of whichare assigned to Owens-Illinois, Inc., assignee of the presentapplication (the disclosure of each of which is incorporated herein byreference and thus forms a part of this application), there aredisclosed several embodiments of heat exchangers, namely recuperatorsand regenerators for use with gas turbine engines for various automotivevehicles, including automobiles, trucks, busses, and the like, togetherwith a full disclosure as to how to make such heat exchangers.

In each embodiment of the disclosures, the regenerative heat exchangercomprises a matrix of integrally fused tubes forming a series of smoothlongitudinal parallel passageways therethrough, wherein the wallsdefining the passageways are nonporous and consist essentially of aninorganic crystalline oxide ceramic material having an average linealcoefficient of thermal expansion of about -12 to +12 × 10⁻ ⁷ /°C.(0°-300°C.). The walls of the passageways also have a maximum thicknessof about 0.03 to 0.002 inches through portions of such walls common toadjacent fused tubes. The ratio of the diameter of the passageways tothe aforementioned thickness is at least 3 and the matrix has an openfrontal area of at least 60%. Furthermore, the matrix walls have athermal conductivity at 400°C. of less than 0.01 cal/cm/sec/cm² /°C.

In the aforementioned U.S. Pat. application Ser. No. 30,859 and in thelater filed application Ser. No. 146,665 which is a continuation-in-partof Ser. No. 30.859, there is disclosed a recuperator comprising a matrixcomprising an assembly of integrally fused tubes arranged in a pluralityof layers of tubes superimposed one above the other in successiveparallel planes, the tubes within each plane being essentially parallelto each other and transverse to the tubes in adjacent layers, the tubesin each layer forming a series of longitudinal parallel passagewaysthrough the matrix. The composition and size of the tubes issubstantially the same as that of the aforementioned regenerator tubes.Because of the arrangement of the layers of tubes in the recuperator,the open frontal or cross-sectional area of each face of the matrixcontaining passageways is at least 32% of the cross-sectional areaacross each face.

In U.S. Pat. application Ser. No. 127,127 filed Mar. 23, 1971, in thenames of Andrew W. Zmuda and Yu K. Pei and entitled "Exhaust Reactor forCombustion Engine " (also assigned to the common assignee) thedisclosure of which is also incorporated herein and forms a part of thisapplication, there is disclosed an integral monolithic exhaust reactorfor a combustion engine including a first reaction zone comprising anelongated tube for receiving the flow of hot glass effluents from acombustion engine. A matrix comprising a plurality of parallel tubesextends longitudinally of and is disposed about the first tubularreaction zone and is bonded thereto. The parallel tubes are bondedtogether in the matrix and form a rigid support for the elongated tube.A plurality of the matrix tubes disposed about the elongated tube haveopen passageways which are in communication with the elongated tube andreceive the flow of gas effluents therefrom. These matrix tubes define asecond reaction zone for the gas effluents and connect with outlet meansto permit passage of gas therefrom.

During their operation, the regenerators, recuperators, and exhaustreactors described in the aforementioned pending patent application aresubjected to extremely high temperatures of about 1,500° F. and more forextended periods of time. When the various components of such devicesare formed of an inorganic crystalline oxide ceramic material, suchmaterial must have thermal stability to withstand such hightemperatures, i.e., its coefficient of thermal expansion, its modulus ofrupture, its structural dimensions, and other properties must not changeto any appreciable extent upon exposure to such high temperatures overextended periods of time.

The coefficient of thermal expansion must be low so that any rapidheating or cooling of the ceramic material components will not cause anybuild-up of thermal stresses and result in breakage of the component.

The modulus of rupture must be high so that such components canwithstand the wear and tear inherent in normal operations of the gasturbines and automotive vehicles. Such strength must be maintained atthe high temperatures of at least 1,500° F. to which such components arenormally subjected.

Heat exchangers, such as regeneraters and recuperators used in gasturbine engines for trucks, operate at temperatures as high as 1,500° F.and must be able to withstand this temperature for at least 1,000 to2,000 hours without shattering during the operation of the truck.Automobile turbine heat exchangers will operate at temperatures as highas 1,900° F., while catalytic and thermal reactors which ensuresubstantially total combustion of exhaust gas effluents of internalcombustion engines will be subjected to temperatures of about 1,000° -1,900° F. Thus, all components of these devices must retain theirstrengths while being subjected to these temperatures over extendedperiods of time and, most importantly, must retain their thermalstability during operation, particularly dimensional stability.Expansion of less than 250 parts per million when exposed totemperatures of at least 1,500° F. for a period of 2,000 hours should bea property of any glass-ceramic component utilized in these heatexchangers and reactors, and preferably such expansion should be lessthan 100 ppm with, of course, the ideal being an expansion of 50 ppm orless at these temperatures over these times.

It has now been found that certain thermally crystallizable glasseshaving certain narrow, critical composition limits within the broad areaof Li₂ O-Al₂ O₃ -SiO₂ and Li₂ O-Al₂ O₃ -SiO₂ -ZnO systems, when shapedinto specific structures and then thermally in situ crystallized to anat least partially crystalline ceramic (also referred to asglass-ceramic), impart to such structures excellent thermal stability athigh temperatures of at least 1,500°F while simultaneously imparting ahigh modulus of rupture. Furthermore, such structures have a coefficientof thermal expansion within the range of -12 to +12×10⁻ ⁷/°C.(0°-700°C). Many of the compositions have a coefficient of thermalexpansion of less than -3 to +3×10⁻ ⁷ /°C. and some are within the rangeof -0.5 to +0.5×10⁻ ⁷ /°C.(0°-700°C).

Thermally crystallizable glass compositions of the invention comingwithin the SiO₂ -Al₂ O₃ -Li₂ O system consist essentially of thefollowing ingredients present within the indicated ranges, expressed inweight percent, and also within the indicated molar ratios.

    ______________________________________                                        Ingredient      Weight Percent                                                ______________________________________                                        SiO.sub.2       55 - 80                                                       Al.sub.2 O.sub.3                                                                              12 - 27                                                       Li.sub.2 O      3.2 - 7.6                                                     Nucleating Agent                                                                              3 - 9                                                         ______________________________________                                    

wherein the nucleating agent is either TiO₂ or a mixture of TiO₂ withZrO₂. Usually, in order to obtain the desired properties, thecompositions will consist of 98-100 weight percent SiO₂ +Al₂ O₃ +Li₂ O+the specified nucleating agents. When the SiO₂ /Al₂ O₃ molar ratio isfrom 4 to less than 5, the Li₂ O/Al₂ O₃ molar ratio is from 0.75 to0.97; when the SiO₂ /Al₂ O₃ molar ratio is from 5 to 7.5, the Li₂ O/Al₂O₃ molar ratio is from 0.65 to 0.97; when the SiO₂ /Al₂ O₃ molar ratiois more than 7.5 and less than 9, then the Li₂ O/Al₂ O₃ molar ratio isfrom 0.8 to 0.97; and when the SiO₂ /Al₂ O₃ molar ratio is more than 9and up to 10, the Li₂ O/Al₂ O₃ molar ratio is from 0.87 to 0.97. Moreusually, the weight percent ranges of SiO₂ and Al₂ O₃ in thesecompositions are 62.8-80 SiO₂ and 12.7-26.7 Al₂ O₃.

When the nucleating agent is a mixture of TiO₂ +ZrO₂ then the ZrO₂should not exceed 3% and preferably should be less than 3% i.e. up to2.8% by weight and preferably at least 0.5% by weight, and the totalamount of TiO₂ plus ZrO₂ is at least 3 weight percent up to about 9weight percent.

While more than about 9 weight percent TiO₂ or total nucleating agentmay be used, there is no advantage in using such large amounts becausethe properties of the resulting glass-ceramics are not improved.Further, raising the TiO₂ levels has the result of raising the thermalexpansion coefficient of the glass-ceramic. It is critical to have atleast 3 weight percent nucleating agent in the composition. When lessthan 3 weight percent total nucleants are present, the nucleation rateis so slow that the heat treatment cycles are excessively long and thestrength of the glass-ceramic which is formed is decreased. When ZrO₂ ispresent in an amount of 3 weight percent or more it raises the liquidusof the glass melt and makes more difficult the processing of the glassinto products.

Refining agents, such as antimony tri-oxide, arsenic oxide, or the like,may be present in the composition in an amount up to about 0.5 weightpercent if such ingredients are deemed necessary for fining the glasses.

In the preferred embodiment of the invention, the thermallycrystallizable glass comes within the following composition rangeswherein the sole essential ingredients, expressed in weight percent, areas follows, and wherein the molar ratios are as indicated:

    Ingredient      Weight Percent                                                ______________________________________                                        SiO.sub.2        63 - 78.5                                                    Al.sub.2 O.sub.3                                                                              13.8 - 23                                                     Li.sub.2 O      3.5 - 6.5                                                     Nucleating Agent                                                                              3 - 9                                                         ______________________________________                                    

wherein the SiO₂ /Al₂ O₃ molar ratio is from 5 to less than 9. Thenucleating agent is as defined above and when the SiO₂ /Al₂ O₃ molarratio is from 7.5 to less than 9, the Li₂ O/Al₂ O₃ molar ratio is from0.87 to 0.97; and when the SiO₂ /Al₂ O₃ molar ratio is from 5 to lessthan 7.5, the Li₂ O/Al₂ O₃ molar ratio is from 0.75 to 0.97.

Representative compositions coming within the scope of the presentinvention are set forth in Table I.

                                      TABLE I                                     __________________________________________________________________________    Composition                                                                   No.     1    2    3    4    5    6    7    8    9    10   11                  __________________________________________________________________________    SiO.sub.2                                                                             78.3 78.90                                                                              75.85                                                                              76.45                                                                              74.8 73.00                                                                              70.85                                                                              70.85                                                                              70.75                                                                              70.80                                                                              70.85               Al.sub.2 O.sub.3                                                                      14.2 13.65                                                                              16.10                                                                              16.20                                                                              17.15                                                                              18.45                                                                              20.05                                                                              20.05                                                                              19.75                                                                              20.05                                                                              20.00               Li.sub.2 O                                                                            3.95 4.00 4.50 3.80 4.50 5.00 5.55 5.40 5.55 5.55 5.45                TiO.sub.2                                                                             1.45 1.45 1.45 1.45 1.45 1.45 1.45 1.40 1.40 1.40 1.40                ZrO.sub.2                                                                             1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.60                Sb.sub.2 O.sub.3                                                                      0.30 0.30 0.30 0.32 0.30 0.30 0.30 0.30 0.30 0.30 0.30                Na.sub.2 O                                                                            0.10      0.10 0.10 0.10 0.10 0.10 0.30 0.10 0.10 0.10                K.sub.2 O                                                 0.20                Fe.sub.2 O.sub.3                                0.45 0.10                     Molar Ratio                                                                   SiO.sub.2 /Al.sub.2 O.sub.3                                                           10.00                                                                              9.80 8.00 8.00 7.40 6.70 6.00 6.00 6.00 6.00 6.00                Molar Ratio                                                                   Li.sub.2 O/Al.sub.2 O.sub.3                                                           0.95 0.97 0.95 0.80 0.90 0.95 0.95 0.95 0.95 0.95 0.95                __________________________________________________________________________    Composition                                                                   No.     12   13   14   15   16   17   18   19   20   21   22                  __________________________________________________________________________    SiO.sub.2                                                                             70.75                                                                              68.00                                                                              71.05                                                                              71.30                                                                              71.50                                                                              69.35                                                                              67.10                                                                              72.00                                                                              68.45                                                                              64.00                                                                              64.85               Al.sub.2 O.sub.3                                                                      20.00                                                                              22.25                                                                              20.10                                                                              20.15                                                                              20.20                                                                              19.60                                                                              18.95                                                                              20.25                                                                              23.25                                                                              25.35                                                                              25.60               Li.sub.2 O                                                                            5.35 6.20 5.30 5.00 4.75 4.55 4.45 4.20 4.75 7.10 6.00                TiO.sub.2                                                                             1.40 1.45 1.45 1.45 1.45 6.00 9.00 1.45 1.45 1.45 1.45                ZrO.sub.2                                                                             1.60 1.60 1.60 1.60 1.60           1.60 1.60 1.60 1.60                Sb.sub.2 O.sub.3                                                                      0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30                Na.sub.2 O                                                                            0.50 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10                K.sub.2 O                                                                     Fe.sub.2 O.sub.3                                                              Molar Ratio                                                                   SiO.sub.2 /Al.sub.2 O.sub.3                                                           6.00 5.20 6.00 6.00 6.00 6.00 6.00 6.00 5.00 4.30 4.30                Molar Ratio                                                                   Li.sub.2 O/Al.sub.2 O.sub.3                                                           0.95 0.95 0.90 0.85 0.80 0.80 0.80 0.70 0.70 0.95 0.80                __________________________________________________________________________

Whenever the glass-ceramic falls outside the range of ingredients setforth above, either the coefficient of thermal expansion is too high orthe dimensional stability at high temperatures, and/or the strengths ofsuch glass-ceramics during high temperature operations are poor. To showthe criticality in the Li₂ O/Al₂ O₃ molar ratio reference is made toTable II wherein several compositions of the invention having differentLi₂ O/Al₂ O₃ ratios were compared with two compositions which wereoutside the range of the invention because of lower ratios. Allcompositions were melted, drawn as 1/4 inch rods from the melt, and theglass rods heated to form glass-ceramic rods.

The glass rods were heated to their nucleating temperature at a rate of300°F per hour, and held at that temperature for 8 hours. They were thenheated to the finishing temperatures set forth in Table II and heldthere for two hours. All rods were then cooled at 25°F per hour to1,500°F and then cooled to room temperature at the furnace rate of about300°F per hour. All compositions had a nucleating temperature of 1,350°Fexcept Compositions 16 and 26which had nucleating temperatures of 1,375°and 1,300°F, respectively.

The foregoing heat treatment schedule was used to form the glass-ceramicrods referred to in all subsequent tables of this application, and thenucleating temperatures of the particular compositions will be set forthin the discussions regarding each table.

The following batch materials were mixed and melted in a furnace to formthe glass of Example 16:

    Ingredients     Parts by Weight                                               ______________________________________                                        Ottawa silica   3580                                                          A-10 Alumina    1016                                                          Titanox         73                                                            Lithium Zirconate                                                                             108.1                                                         Sodium Antimonate                                                                             20.5                                                          Lithium Carbonate                                                                             488                                                           Lithium Chloride                                                                              47.8                                                          Lithium Nitrate 27.6                                                          ______________________________________                                    

The sodium antimonate, lithium chloride and lithium nitrate are presentas refining agents for the glass melt. The batch was melted at 2,970°F.for a period of 23 hours and mechanically stirred during this time. Fromthe disclosure of the batch of the foregoing Example, those in the glassart can readily make the glasses coming within the present embodiment ofthe invention.

                                      TABLE II                                    __________________________________________________________________________    Composition No.                                                                          23    24                                                                              7      14     15     16     19    25   26                  __________________________________________________________________________    Ingredients                                                                   (Moles)                                                                        Li.sub.2 O                                                                              1.05  1.0                                                                             0.95   0.9    0.85   0.8    0.7   0.6  0.5                  Al.sub.2 O.sub.3                                                                        1     1 1      1      1      1      1     1    1                    SiO.sub.2 6     6 6      6      6      6      6     6    6                   Heat Treatment                                                                Temp. °F(Time,Hrs)                                                                2220(2)                                                                             --*                                                                             2100(2)                                                                              2200(2)                                                                              2200(2)                                                                              2200(2)                                                                              2100(2)                                                                             2200(2)                                                                            2200(2)             Dimensional Sta-                                                              bility, change in                                                             ppm:                                                                          at 1500°F                                                                         +490(10)                                                                              +27(100)             +35(100)                                                                             -13(250)                        (Time,Hrs)                                                                   at 1900°F   -196(2000)                                                                           -104(2000)                                                                           -182(2000)                                                                           +205(2000)                                                                           -104(500)  +76(100)             (Time,Hrs)                                                                   Modulus of Rupture                                                            (psi, Room Temp.)                                                                        14,600  11,300 10,500 10,700 11,500 12,200     4300                Coefficient of                                                                Thermal Expansion                                                             ×10.sup.+.sup.7 (0-700°C.)                                                  +3.8    +1.6   +1.0   +1.1   +1.6   +1.8       +8.8                __________________________________________________________________________     *Samples broke. At Li.sub.2 O/Al.sub.2 O.sub.3 molar ratios of 1, the         resulting glass could not be heat treated to a glass-ceramic since the        samples all broke during the heat treating steps.                        

The Li₂ O/Al₂ O₃ molar ratios of 1.05 in Composition 23 resulted in theglass-ceramics having very poor dimensional stability at 1,500°F. Theexpansion of +490 parts per million after only 10 hours at 1,500° F.showed that with longer times the expansion would be considerablygreater and the glass-ceramic would not be satisfactory where theproduct made therefrom would be subjected to temperatures of at least1,500°F. for extended periods of time. Compositions 24 also wasunacceptable since the glass-ceramic broke during the heat treatment.When the Li₂ O/Al₂ O₃ molar ratio was within the range of 0.95 to 0.70,the resulting glass ceramic had good high temperature propertiesincluding good dimensional stability at 1,900° F. However, when themolar ratio of Li₂ O/Al₂ O₃ was as low as 0.6 and lower as inCompositions 25 and 26, the resulting glass-ceramics had very lowstrengths. The samples of Example 25 were very weak at the intermediateheat treatment step of 1,800° to 2,000° F. and were very unsatisfactory.Compositions of the invention, however, had modulus of rupture values inexcess of 10,000 psi and are suitable for use at high temperatures overlong periods of time because of these strengths. The glass-ceramic ofComposition 26 had a coefficient of thermal expansion of 18.2 × 10⁻ ⁷(0°-300°C). Such compositions are unsuitable for the purposes of thisinvention.

Compositions 20, 22, and 29, which are representative of those comingwithin the scope of the present invention, were compared with othercompositions falling outside of the invention because of too low asilica content and a resulting low SiO₂ /Al₂ O₃ ratio. Again, thecompositions were melted, glass rods of one-fourth inch diameter weredrawn out therefrom and then heat treated according to the schedule setforth in connection with the Table II compositions and the finishingtemperatures given in Table III. The nucleating temperatures were 1,250°F. for Composition 28; 1,300° F. for Compositions 22 and 27; and 1,350°F. for Compositions 20 and 29. Various properties of the resultingglass-ceramics were then measured.

                                      TABLE III                                   __________________________________________________________________________    Composition No.                                                                             22     27     28    20    29                                    __________________________________________________________________________    Ingredients                                                                    (Moles)                                                                        Li.sub.2 O 0.95   0.95   0.7   0.7   0.8                                      Al.sub.2 O.sub.3                                                                         1      1      1     1     1.0                                      SiO.sub.2  4.3    3.2    4.3   5     4.3                                    Heat Treatment                                                                Temp.°F(Time,Hrs.)                                                                  2100(2)                                                                              2100(2)                                                                              2100(2)                                                                             2200(2)                                                                             2100(2)                                Dimensional Stability                                                         Parts Per Million                                                             (Time, Hrs.)                                                                  at 1500°F (hrs)                                                                     -15(2000)                                                                            -298(100)                                                                            -79(250)                                                                            -8(250)                                      at 1900°F (hrs)           +37(250)                                     Modulus of Rupture                                                            (psi room temp)                                                                            11,200        6,100 13,800                                                                              11,300                                 Coefficient of                                                                Thermal Expansion                                                             α ×10.sup.+.sup.7 (0-700°C)                                             +6.8   +5.3   -5.9  +4.2  7.1                                    __________________________________________________________________________

Composition 22 gives excellent high temperature and good strengthproperties. When the SiO₂ /Al₂ O₃ molar ratio is 3.2 as in composition27, however, the forming properties of the glass are very poor becauseof the low silica content and the resulting glass-ceramic also has verypoor dimensional stability at 1,500°F. The expansion after 100 hrs.indicates that the glass-ceramics will expand considerably more afterlonger times at this high temperature, thus making it veryunsatisfactory for purposes of this invention. As seen from Composition28, a Li₂ O/Al₂ O₃ molar ratio of 0.7 is too low when the SiO₂ /Al₂ O₃ratio is 4.3 and results in a glass ceramic having very low strengthvalues. However, with the same low 0.7 Li₂ O/Al₂ O₃ molar ratio, a goodglass ceramic is obtained when the SiO₂ /Al₂ O₃ ratio is 5, as shown byComposition 20.

Composition 29, which has a lower Li₂ O/Al₂ O₃ molar ratio thanComposition 22, still produced a glass ceramic having a high modulus ofrupture and a low expansion coefficient, comparable to that ofComposition 22.

The effect of Li₂ O on compositions having a high SiO₂ /Al₂ O₃ ratio wasascertained and the results are set forth in Table IV. Again, eachcomposition had its batch ingredients melted, and a rod having aone-fourth inch diameter was drawn therefrom and heat treated accordingto the nucleating and crystallization rates set forth above, using thefinishing temperature set forth in Table IV. The nucleating temperaturewas 1,350°F. for Compositions 3, 30 and 31 and 1,375°F for Compositions1 and 4.

                                      TABLE IV                                    __________________________________________________________________________    Composition No.                                                                             4     3     1     30    31                                      __________________________________________________________________________    Ingredients                                                                    (Moles)                                                                        Li.sub.2 O  0.8   0.95  0.95  0.6   0.8                                       Al.sub.2 O.sub.3                                                                          1     1     1     1     1                                         SiO.sub.2   8     8     10    10    10                                      Heat Treatment                                                                Temp.°F (Time,Hrs.)                                                                 2100(2)                                                                             2100(2)                                                                             2100(2)                                                                             2100(2)                                                                             2100(2)                                  Dimensional Stability                                                         Parts Per Million                                                             (Time, Hrs.)                                                                  at 1500°F (hrs)                                                                     +71(100)                                                                            +96(100)                                                                            +48(100)    +14(250)                                 at 1900°F (hrs)         -76(250)                                       Strengths    8,700 11,600                                                                              10,700                                                                              5,100 5,100                                    (psi, room temp)                                                              Coefficient of                                                                Thermal Expansion                                                                           -2.7  -2.8  -4.9 +12.7 +12.4                                    α × 10.sup.+.sup.7 (0-700°C)                               __________________________________________________________________________

Compositions 1, 3 and 4 exhibit acceptable high temperature properties.On the other hand, Compositions 30 and 31 are not suitable for thepurpose of this invention because of their high thermal expansionproperties and their low strengths. Thus, the 0.7 Li₂ O/Al₂ O₃ ratiofound acceptable at other SiO₂ /Al₂ O₃ molar ratios is unacceptable at aSiO₂ /Al₂ O₃ ratio of 10. Further, the glass-ceramic of Composition 30had a coefficient of thermal expansion of 200.4 × 10⁻ ⁷ (0°-300°C) whilethat of Composition 31 had a coefficient of 21.1 over the same 0°-300°Crange. Both compositions are unsuitable for purposes of this inventionbecause of these very poor expansion properties.

It should be emphasized that for purposes of this invention, thethermally crystallizable glass compositions must consist of the threeessential ingredients, SiO₂, Al₂ O₃ and Li₂ O plus the nucleatingagent(s), all falling within the narrow critical ranges set forth above,both as to weight percent of each ingredient and to molar ratios of Li₂O/Al₂ O₃ and SiO₂ /Al₂ O₃. Ingredients and impurities such as Na₂ O, K₂O, fluorides and the like which are present in substantial amounts inthe prior art Li₂ O-SiO₂ -Al₂ O₃ compositions cannot be tolerated abovecertain very minor levels and preferably should be omitted altogetherfrom the compositions of the present invention. The effect of Na₂ O, K₂O and F on the dimensional stability of the glass-ceramic compositionsis set forth in Table V. All of the glass ceramics were formed fromglass rods which were then heat treated at 2,100°F for 2 hours.

                                      TABLE V                                     __________________________________________________________________________    Composition No.                                                                            7   8   12  11  32  33  9   10  34  35                           __________________________________________________________________________    Ingredients                                                                    (Wt.%)                                                                         Na.sub.2 O 0.1 0.3 0.5 0.1 0.5 0.8 0.1 0.1 0.1 0.1                            K.sub.2 O  --  --  --  0.2 0.2 0.4 --  --  --  --                             Fe.sub.2 O.sub.3                                                                         --  --  --  --  --  --  0.45                                                                              0.1 --  --                             F.sub.2    --  --  --  --  --  --  --  --  0.3 0.2                          Dimensional Stability                                                         Parts Per Million                                                             (Time, Hrs.)                                                                  at 1500°F-100 hrs.                                                                  +27 0   --  --   +613                                                                              +307                                                                             -19 -9   +750                                                                             *                            at 1500°F-2000 hrs.                                                                  -130                                                                             -11 -6  -3  --  --  +5  -74 --  --                           at 1900°F-2000 hrs.                                                                 -32  +330                                                                              +434                                                                              +987                                                                             --  --   +168                                                                             -78 --  --                           __________________________________________________________________________     *Composition 35 was unsatisfactory since it crazed on its surface, no         matter what heat treatment was used.                                     

As evident from the data in Table V, Na₂ O and K₂ O have a verydetrimental effect on the dimensional stability at high temperatures ofthe compositions of the present invention. For example, compositions 32and 33 have a 0.95:1:6 molar ratio of Li₂ O-Al₂ O₃ -SiO₂ and containdifferent levels of Na₂ O+K₂ O, namely 0.7 and 1,2 total weight percent,respectively. While compositions 8, 11 and 12 are dimensionally unstableat 1,900°F, they do have satisfactory stability at 1,500°F for 2,000hours and can be used in those devices wherein a temperaturesubstantially over 1,500°F will not be encountered during normaloperation, such as in a regenerator for a gas turbine engine used fortrucks.

To achieve good dimensional stability at 1,500°F, the Na₂ O should notbe present in an amount more than 0.5 weight percent and the K₂ O shouldnot be present in an amount of more than 0.5 weight percent, nor shouldthe combined Na₂ O+K₂ O exceed 0.5 weight percent. To obtain goodstability at temperatures of 1,900°F no more than about 0.25 percent Na₂O or K₂ O can be present, either by themselves or as a mixture.

Fe₂ O₃ when present as an impurity in an amount of less than 0.5 weightpercent has no detrimental effect on the compositions of the invention.However, fluoride ion, which is known by the art to increase thestrength of glass-ceramic compositions when present in small amounts,adversely affects the properties of the compositions of the presentinvention. Composition 35 contained 0.2 weight percent F and could notbe heat treated without the formation of surface scale because of thecrazing which took place. Composition 34 had very poor dimensionalstability, namely a change of 750 ppm after only 100 hours at 1,500°F.That the effect was due to the F, even though ZnO was present in thecomposition will become evident from the further disclosure of anotherembodiment of the invention where it will be demonstrated that ZnO doesnot have a detrimental effect on dimensional stability at 1,500°F, but,instead, makes the glass-ceramic stronger. Fluorine should be kept toless than 0.1 weight percent and preferably is absent.

It is critical in this first embodiment of the invention to have thecrystallizable glass and the glass-ceramic consist essentially solely ofthe three essential ingredients plus the nucleating agent coming withinthe weight amounts and molar ratios defined above. It is imperative thatthe inpurities be kept to an absolute minimum. While no impuritiesshould be present it is recognized that it is almost impossible to makeglass without some impurities unless only pure materials were used inthe batch. This is not possible on any commercial scale because of thecosts. However, care should be maintained in selecting batch materialsfor making the glasses to insure that impurities in the resultingglasses are kept to as low a figure as possible. It has been found thatwhen petalite is used as a batch ingredient, for instance, that theresulting glass-ceramics do not have the desired properties of theglass-ceramics of the present invention. Accordingly, petalite cannot beused as a batch ingredient in making the glasses and glass-ceramics ofthe present invention.

In such other embodiment of the invention it has been found thatthermally crystallizable glass compositions consisting essentially ofSiO₂, Al₂ O₃, Li₂ O and ZnO, wherein each ingredient is within a narrowcritical range and also within certain critical molar ratios, can beformed into articles which when heat treated to thermally in situcrystallize them to an at least partially crystalline ceramic, alsoreferred to as a glass-ceramic, exhibit good thermal stability, goodflexural strengths and low coefficients of thermal expansion. Thepresence of the ZnO vastly improves the strengths of the glass-ceramicsto almost double the strengths of the compositions of the firstembodiment of the invention discussed above. Strengths within the rangeof about 13,000-20,000 psi and more can be readily obtained.

Such compositions coming within the scope of this second embodiment ofthe invention consist essentially of the following ingredients presentwithin the indicated weight percent ranges and also within the specificmolar ratios:

    Ingredient         Weight Percent                                             ______________________________________                                        SiO.sub.2          52 - 78.4                                                  Al.sub.2 O.sub.3   12.8 - 27.5                                                Li.sub.2 O         2.4 - 7                                                    ZnO                0.25 - 6.7                                                 Nucleating Agent   3 - 9                                                      ______________________________________                                    

wherein the (ZnO + Li₂ O/Al₂ O₃) molar ratio is ≦1 and

when the (SiO₂ /Al₂ O₃) molar ratio is from 4 to less than 5

then the (Li₂ O/Al₂ O₃) molar ratio is from 0.55-0.91

and the (ZnO/Li₂ O) molar ratio is from 0.06 to 0.5;

when the (SiO₂ /Al₂ O₃) molar ratio is from 5 to 8;

then the (Li₂ O/Al₂ O₃) molar ratio is from 0.55 to 0.93

and the (ZnO/Li₂ O) molar ratio is from 0.04 to 0.5; and

when the (SiO₂ /Al₂ O₃) molar ratio is from more than 8 and up to 9,

then the (Li₂ O/Al₂ O₃) molar ratio is from 0.8 to 0.93

and the (ZnO/Li₂ O) molar ratio is from 0.04 to 0.5. Usually, in orderto obtain the desired properties, the compositions will consist of98-100 weight percent SiO₂ + Al₂ O₃ + Li₂ O + ZnO + the specifiednucleating agents.

The nucleating agent can be TiO₂, ZrO₂ or a mixture of TiO₂ and ZrO₂. IfTiO₂ alone is used, it should be in an amount of at least 3.2 weightpercent. If ZrO₂ is used with TiO₂, it should not exceed 3% andpreferably should be less than 3%, i.e. from about 0.5 to about 2.8weight percent. In the latter event, sufficient TiO₂ must be present toat least give a total of 3 weight percent nucleating agent mixture inthe composition.

Representative compositions coming within the scope of the secondembodiment of the invention are set forth in Table VI.

                                      TABLE VI                                    __________________________________________________________________________    Composition No.                                                                             36   37   38   39   40   41   42   43                           __________________________________________________________________________    Ingredients                                                                   (Weight Percent)                                                              SiO.sub.2     70.25                                                                              73.00                                                                              72.00                                                                              72.60                                                                              70.2 71.15                                                                              72.05                                                                              70.25                        Al.sub.2 O.sub.3                                                                            19.85                                                                              17.65                                                                              18.80                                                                              18.85                                                                              18.4 18.50                                                                              18.85                                                                              19.85                        ZnO           1.70 1.70 1.80 1.70 3.7  2.50 1.0  1.70                         Li.sub.2 O    4.65 4.15 3.85 3.30 4.3  4.30 4.60 4.65                         TiO.sub.2     1.45 1.40 1.45 1.45 1.45 1.45 1.40 2.40                         ZrO.sub.2     1.60 1.60 1.60 1.60 1.60 1.60 1.60 0.60                         Sb.sub.2 O.sub.3                                                                            0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30                         Na.sub.2 O    0.10 0.10 0.10 0.10 0.1  0.10 0.10 0.10                         K.sub.2 O     --   --   --   --   --   --   --   --                           Fe.sub.2 O.sub.3                                                                            --   --   --   --   --   --   --   --                           Molar Ratios                                                                  SiO.sub.2 Al.sub.2 O.sub.3                                                                  6.0  7.0  6.5  6.5  --   6.5  6.5  6.0                          Li.sub.2 O/Al.sub.2 O.sub.3                                                                 0.8  0.8  0.7  0.6  --   0.8  0.76 0.8                          ZnO/Li.sub.2 O                                                                              0.14 0.14 0.16 0.18 --   0.21  .086                                                                              0.14                         ZnO + Li.sub.2 O/Al.sub.2 O.sub.3                                                           0.91 0.91 0.81 0.71 --   0.97 0.84 0.91                         __________________________________________________________________________    Composition No.                                                                             44   45   45   47   48   49   50   51                           __________________________________________________________________________    Ingredients                                                                   (Weight Percent)                                                              SiO.sub.2     72.95                                                                              72.85                                                                              70.75                                                                              70.75                                                                              73.25                                                                              72.70                                                                              68.50                                                                              61.65                        Al.sub.2 O.sub.3                                                                            17.30                                                                              17.60                                                                              17.10                                                                              17.10                                                                              19.00                                                                              18.85                                                                              16.55                                                                              25.50                        ZnO           1.70 1.70 1.65 1.65 0.85 1.70 1.60 1.80                         Li.sub.2 O    4.10 4.15 4.00 4.00 3.35 3.30 3.85 4.50                         TiO.sub.2     1.40 1.40 6.0  3.00 1.45 1.45 9.0  1.45                         ZrO.sub.2     1.60 1.60 --   3.00 1.60 1.60 --   1.60                         Sb.sub.2 O.sub. 3                                                                           0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30                         Na.sub.2 O    0.10 0.10 0.10 0.10 0.10 --   0.10 0.10                         K.sub. 2 O    --   0.20 --   --   --   --   --   --                           Fe.sub.2 O.sub.3                                                                            0.45 --   --   --   --   --   --   --                           Molar Ratios                                                                  SiO.sub.2 /Al.sub.2 O.sub.3                                                                 7.0  7.0  7.0  7.0  6.5  6.5  7.0  4.3                          Li.sub.2 O/Al.sub.2 O.sub.3                                                                 0.8  0.8  0.8  0.8  0.6  0.6  0.8  0.6                          ZnO/Li.sub.2 O                                                                              0.14 0.14 0.14 0.14 0.08 0.18 0.14 0.13                         ZnO + Li.sub.2 O/Al.sub.2 O.sub.3                                                           0.91 0.91 0.91 0.91 0.65 0.71 0.91 0.68                         __________________________________________________________________________    Composition No.                                                                             52   53   54   55   56   57   58   59A                          __________________________________________________________________________    Ingredients                                                                   (Weight Percent)                                                              SiO.sub.2     70.42                                                                              73.35                                                                              59.80                                                                              75.30                                                                              73.55                                                                              75.85                                                                              72.30                                                                              72.55                        Al.sub.2 O.sub.3                                                                            18.40                                                                              19.15                                                                              29.00                                                                              16.00                                                                              19.20                                                                              16.10                                                                              18.85                                                                              18.90                        ZnO           4.40 0.60 2.55 1.40 0.35 0.70 0.85 0.60                         Li.sub.2 O    3.23 3.35 5.10 3.75 3.35 3.80 4.45 4.40                         TiO.sub.2     1.45 1.45 1.45 1.45 1.45 1.45 1.45 1.45                         ZrO.sub.2     1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.60                         Sb.sub.2 O.sub.3                                                                            0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30                         Na.sub.2 O    0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10                         K.sub.2 O     --   --   --   --   --   --   --   --                           Fe.sub.2 O.sub.3                                                                            --   --   --   --   --   --   --   --                           Molar Ratios                                                                  SiO.sub.2 /Al.sub.2 O.sub.3                                                                 6.5  6.5  3.5  8    6.5  8    6.5  6.5                          Li.sub.2 O/Al.sub.2 O.sub.3                                                                 0.6  0.6  0.6  0.8  0.6  0.8  0.8  0.8                          ZnO/Li.sub. 2 O                                                                             0.5  0.06 0.18 0.14 0.04 0.07 0.07 0.05                         ZnO + Li.sub.2 O/Al.sub.2 O.sub.3                                                           0.90 0.64 0.71 0.91 0.62 0.86 0.86 0.84                         __________________________________________________________________________

The Li₂ O-ZnO-Al₂ O₃ -SiO₂ compositions of this embodiment of theinvention have better heat treatment characteristics than theaforementioned Li₂ O-Al₂ O₃ -SiO₂ compositions of the invention. At heattreatment temperatures of 2,000°-2,200°F, the latter compositionsundergo a dimensional expansion which is caused by grain growth and thedevelopment of voids or pores at the grain boundaries. The change inlength during heat treatment of Compositions 16 and 37 is shown inFIG. 1. Both compositions undergo an initial shrinkage and thisshrinkage accompanies the crystallization of the high quartz solidsolution phases in the temperature range of 1,350°F to 1,600°F. Attemperatures between 1,600°F and 2,000°F, the high quartz phaseundergoes a solid state transformation to the keatite solid solutionphase. This phase change in Composition 37 is accompanied by a 0.3%lineal expansion at about 1,725°F. At temperatures above 2,000°F,Composition 16 expands by about 2.5% but Composition 37, which containsZnO, does not change.

The behavior of these compositions at high temperatures gives theZnO-containing compositions of the invention an advantage over thecompositions of the invention which do not contain ZnO. With the lattercompositions, the heating rates in the heat treatment cycles must beslowed down in temperature ranges where dimensional changes occur,particularly when an article such as a matrix or assembly for aregenerator, recuperator, thermal exhaust reactor and the like, is beingheat treated to thermally in situ crystallize it to form the integralmonolithic glass ceramic structure. If the product being heat treatedhas a temperature gradient, parts of it will tend to change dimensionsbefore other of its parts and mechanical stress resulting in fracturewill occur. Thus it takes about a week to heat treat and thuscrystallize a product having a substantial thickness of several inchesor more made from Composition 16 and less than two days to heat treatand crystallize the same product made from Composition 37.

Furthermore, it is easier to predict and control the dimensions of aproduct made from a glass composition which undergoes very smalldimensional changes in the final stage of the heat treatment cycle as itbecomes a glass-ceramic product.

Another important advantage of the ZnO-containing compositions of thepresent invention is that they are significantly stronger than thecompositions of the invention which do not contain ZnO. The roomtemperature modulus of rupture of the former is at least 13,000 and isoften 20,000 psi or higher as compared to approximately 10,000 psi forthe latter.

Na₂ O, K₂ O, F₂ and other impurities, when present in the ZnO-containingcompositions of the present embodiment of the invention also adverselyaffect the dimensional stability of the glass-ceramic formed therefrom.Thus, as little as 0.3 weight percent Na₂ O causes poor dimensionalstability of the glass-ceramic, namely an expansion of +631 ppm whenheld at 1,500°F for 250 hours for Composition 59A. Na₂ O+K₂ O in anamount of about 0.3 weight percent also adversely affects the stabilityas does K₂ O per se when present in such an amount. Thus, an importantand essential feature of these compositions is that the weight percentof Na₂ O and K₂ O, either alone or combined be no more than 0.2 weightpercent. Fe₂ O₃, when present as an impurity in an amount less than 0.5weight percent, has substantially no effect on the stability propertiesof the glass-ceramic. F, on the other hand, even when present in anamount as low as 0.2 weight percent, imparted a 750 ppm change inComposition 34 after 100 hours at 1,500°F. See Table V, supra. Thus,fluorine should be present in amounts less than 0.1 percent, andpreferably not at all.

To show the effect of the Li₂ O/Al₂ O₃ molar ratio on the properties ofthe compositions, five glasses were melted, drawn as 1/4 inch rods andheat treated in accordance with the schedule set forth above withrespect to the Table II compositions and where the finishingtemperatures are set forth in the following Table VII. All compositionshad a nucleating temperature of 1,350°F except for Composition 60, whichwas 1,300°F. Composition 37 was prepared from the followingbatch:Ingredient Parts byWeight______________________________________Ottawa silica 3612A-10Alumina 888Titanox 70.5Sodium Antimonate 20.5Lithium Carbonate487Lithium Chloride 48Lithium Nitrate 28Zinc Oxide 70.6Zinc ZirconiumSilicate 176.5______________________________________

The batch was melted at 2,980°F for twenty-two hours while beingmechanically stirred. Other compositions coming within the scope of thisembodiment of the invention were similarly made, except that the amountsof the ingredients varied accordingly. Those in the art will know how tomake all of the compositions of the invention from the foregoingdisclosure.

Dimensional stability and strengths of each glass-ceramic were thenmeasured.

                                      TABLE VII                                   __________________________________________________________________________    Composition No.                                                                            37      38      39      60                                       __________________________________________________________________________    Ingredients                                                                    (Moles)                                                                       Li.sub.2 O  0.8     0.7     0.6     0.4                                       Al.sub.2 O.sub.3                                                                          1       1       1       1                                         SiO.sub.2   7.0     6.5     6.5     6.5                                       ZnO         .11     .11     .11     .11                                      Heat Treatment                                                                Temp.°F(Time,Hrs.)                                                                  2100(2) 2200(2) 2200(2) 2200(2)                                  Dimensional Stability                                                         Parts Per Million                                                             (Time, Hrs.)                                                                  at 1500°F (hrs)                                                                     -157(2000)                                                                            -13.1(25)                                                                             -164(1000)                                                                            48(500)                                  at 1900°F (hrs)                                                                     --      --      -170(250 hr)                                                                          -174(250)                                                                     -261(500)                                Modulus of Rupture                                                                         16,100  16,700  16,600  14,100                                    (psi room temp)                                                              Coefficient of                                                                Thermal Expansion                                                                          -2.7    -1.9    -1.5    35.8                                     α × 10.sup.+.sup.7 (0-700°C)                               __________________________________________________________________________

Compositions 37, 38 and 39 have excellent stability after beingsubjected to the high temperatures for extended periods of time and alsohave high strengths. Composition 60 which has a low Li₂ O/Al₂ O₃ molarratio has a very high coefficient of expansion which makes it completelyunsatisfactory for purposes of the invention.

To show the effect of ZnO to the basic properties of the glass-ceramic,the following glass-ceramics were prepared from correspondingcrystallizable glasses, drawn into 1/4 inch rods and heat treated inaccordance with the schedule set forth above with respect to the TableII compositions. The finishing temperatures are set forth in Table VIII.The nucleation temperatures are as follows: 1,250°F for Compositions 40and 41; 1,300°F for Composition 42 and 1,350°F for the remaining fourcompositions.

                                      TABLE VIII                                  __________________________________________________________________________    Composition No.                                                                            40    41    37     42     61     62     63                       __________________________________________________________________________    Ingredients                                                                    (Moles)                                                                       Li.sub.2 O  0.8   0.8   0.8    0.76   0.8    0.8    0.8                       Al.sub.2 O.sub.3                                                                          1     1     1      1      1      1      1                         SiO.sub.2   6.5   6.5   7.0    6.5    6.5    6.5    6.5                       ZnO         0.25  0.17  0.11   0.07   0.04   0.02   0.01                      Molar Ratio                                                                   ZnO/Li.sub.2 O                                                                            0.31  0.21  0.14   0.09   0.05   .025   .0125                    Heat Treatment                                                                             2100(2)                                                                             2100(2)                                                                             2100(2)                                                                              2200(2)                                                                              2100(2)                                                                              2100(2)                                                                              2100(2)                  Temp.°F (Time,Hrs.)                                                    Dimensional Stability                                                         Parts Per Million                                                             (Time, Hrs.)                                                                  at 1500°F (hrs)                                                                     -95(500)                                                                            -32(100)                                                                            -157(2000)                                                                           -74(2000)                                                                            -147(500)                                                                            +4(250)                                                                              -9(250)                  at 1900°F (hrs)                                                                     --    --    --     --     -119(1000)                                                                           +156(500)                                                                            -147(250)                Modulus of Rupture                                                                         20,100                                                                              13,800                                                                              16,100 15,100 13,400 10,100 9,900                     (psi room temp)                                                              Coefficient of                                                                Thermal Expansion                                                                          -1    -.2   -2.7   -1.0   +6.0   -0.1   -0.6                     α × 10.sup.+.sup.7 (0-700°C)                               __________________________________________________________________________

From the above table it can be seen that compositions 37, 40, 41, 42 and61, which have a SiO₂ /Al₂ O₃ ratio of 0.8 and less have a molar ratioof ZnO/Li₂ O within the range of 0.04 to 0.5 and thus the presence ofthe ZnO affects and increases the modulus of rupture properties of theglass-ceramics. However, when the ZnO/Li₂ O molar ratio is outside ofthe range of 0.04 to 0.5, the resulting glass ceramics, while they havegood high temperature properties, are no better than glass-ceramiccompositions having no ZnO. Thus, in order to achieve the increase inmodulus of rupture, the amount of ZnO must come within the criticalranges set forth above, both as to weight percent and as to molarratios.

The most usual compositions of this embodiment of the invention are asset forth hereinbefore but with the weight percent ranges of SiO₂ Al₂O₃, Li₂ O and ZnO in the following narrow ranges:

    Ingredient      Weight Percent                                                ______________________________________                                        SiO.sub.2       58.2 - 78.4                                                   Al.sub.2 O.sub.3                                                                              13.7 - 26.5                                                   Li.sub.2 O       3.2 -  6.9                                                   ZnO              0.3 -  6.5                                                   ______________________________________                                    

The effect resulting from too high a SiO₂ level in the compositions isseen from Table IX. The nucleation temperature for Compositions 55, 57and 65 were 1,350°F and the finishing temperatures and times are setforth in Table IX. Again, the heat treating schedule was the same asthat given for the Table II compositions.

                  TABLE IX                                                        ______________________________________                                        Composition No.                                                                            64     65         55     57                                      ______________________________________                                        Ingredients                                                                    (Moles)                                                                       Li.sub.2 O  0.6    0.8        0.8    0.8                                      Al.sub.2 O.sub.3                                                                          1      1          1      1                                        SiO.sub.2   10     10         8      8                                        ZnO         0.11   0.11       0.11   0.056                                   Heat Treatment                                                                Temp.°F(Time,Hrs.)                                                                  --     2100(2)    2200(2)                                                                              2200(2)                                 Dimensional Stability                                                         Parts Per Million                                                             (Time,Hrs.)                                                                   at 1500°F                                                                           --     +49(25)    20(10) --                                      at 1900°F                                                                           --     155(10)    +23(10)                                                                              --                                                          low strength                                              Modulus of Rupture                                                             (psi room temp)                                                                           --     7,000      21,000 20,800                                  Coefficient of                                                                Thermal Expansion                                                                          --     +11        -4.6   -4.0                                    α × 10.sup.+.sup.7 (0-700°C)                               Coefficient of                                                                Thermal Expansion   +31        --     --                                      α × 10.sup.+.sup.7 (0-300°C)                               ______________________________________                                    

Composition 64 could not be properly crystallized and was a very weakproduct having a modulus of rupture of 2,000°-3,000 psi. Composition 65has a very low modulus of rupture and a high coefficient of thermalexpansion, both over the 0°-700°C and the 0°-300°C ranges. When the SiO₂/Al₂ O₃ molar ratio was 8, however, as in Compositions 55 and 57, thestrengths of the glass-ceramics tripled over that of composition 65 theexpansion coefficients became very low.

The effect resulting from too low a SiO₂ level in the compositions isseen from Table X. While Compositions 51 and 54 have good hightemperature properties, where the Li₂ O/Al₂ O₃ ratio is as low as 0.6with low SiO₂ levels, the SiO₂ /Al₂ O₃ should not be lower than 4,because of the very poor forming properties of the glass. Composition66, on the other hand, did not crystallize and broke during the heattreatment. The nucleation temperature for Compositions 51 and 54 was1,350°F.

                  TABLE X                                                         ______________________________________                                        Composition No.                                                                              51        54        66                                         ______________________________________                                        Ingredients                                                                    (Moles)                                                                       Li.sub.2 O    0.6       0.6       0.6                                         Al.sub.2 O.sub.3                                                                            1.0       1.0       1.0                                         SiO.sub.2     4.3       3.5       4.3                                         ZnO           0.08      0.11      0.024                                      Heat Treatment 2200(2)   2200      --                                         Temp.°F(Time,Hrs.)                                                     Dimensional Stability                                                         Parts Per Million                                                             (Time, Hrs.)                                                                  at 1500°F                                                                             -39(250)  -31(250   --                                         at 1900°F                                                                             --        -88(250)  --                                         Modulus of Rupture                                                                           13,400    16,000    --                                          (psi room temp)                                                              Coefficient of +5.8      +10.8     --                                         Thermal Expansion                                                             α × 10.sup.+.sup.7 (0-700°C)                               ______________________________________                                    

In all of the work set forth in the foregoing tables, five rods weredrawn from each melt and the dimensional stability measurements arebased on the average stability of the five rods. For the modulus ofrupture tests, 10 rods were drawn from each melt and the strengths setforth in the tables is the average strength of the ten rods. The modulusof rupture tests were performed on a Dillon Universal Tester, Low RangeHead, having a 4 point loading with a support span of 4 inches and aloading span of three-fourths inch. The rods were each 51/2 inches inlength and the loading rate adjusted so that the samples broke between36 and 48 seconds. The 10 samples were abraded by tumbling for 15minutes in a ball mill jar rotating at 75 rpm and containing 250 gramsof 240 grit SiC.

Compositions 23-35 and 59-66, inclusive, referred to in the above tableshave the following compositions expressed in weight percents:

                                      TABLE XI                                    __________________________________________________________________________    Composition Nos.                                                                        24   23   25   26   27   28   29                                    __________________________________________________________________________    Ingredients                                                                   SiO.sub.2 70.7 70.5 72.4 72.9 61   65.4 64.9                                  Al.sub.2 O.sub.3                                                                        20   19.9 20.5 20.6 27.8 25.8 25.6                                  ZnO       --   --   --   --   --   --   --                                    Li.sub.2 O                                                                              5.9  6.1  3.6  3    7.7  5.3  6                                     TiO.sub.2 1.45 1.45 1.45 1.45 1.45 1.45 1.45                                  ZrO.sub.2 1.6  1.6  1.6  1.6  1.6  1.6  1.6                                   Sb.sub.2 O.sub.3                                                                        0.3  0.3  0.3  0.3  0.3  0.3  0.3                                   Na.sub.2 O                                                                              0.1  0.1  0.1  0.1  0.1  0.1  0.1                                   K.sub.2 O --   --   --   --   --   --   --                                    Fe.sub.2 O.sub.3                                                                        --   --   --   --   --   --   --                                    __________________________________________________________________________    Composition Nos.                                                                        30   31   32   33   34   35   59                                    __________________________________________________________________________    Ingredients                                                                   SiO.sub.2 80.5 79.8 70.6 70.2 70.6 71.3 72.8                                  Al.sub.2 O.sub.3                                                                        13.6 13.5 20   19.8 20   20.2 17.6                                  ZnO       --   --   --   --   1.7  --   1.7                                   Li.sub.2 O                                                                              2.4  3.15 5.3  5.3  3.8  4.8  4.2                                   TiO.sub.2 1.45 1.45 1.45 1.45 1.45 1.45 1.4                                   ZrO.sub.2 1.6  1.60 1.6  1.6  1.6  1.60 1.6                                   Sb.sub.2 O.sub.3                                                                        0.3  0.3  0.3  0.3  0.3  0.3  0.3                                   Na.sub.2 O                                                                              0.1  0.1  0.5  0.8  0.1  0.1  0.3                                   K.sub.2 O --   --   0.2  0.4  --   --   --                                    Fe.sub.2 O.sub.3                                                                        --   --   --   --   --   --   --                                    F.sub.2   --   --   --   --   0.3  0.2  --                                    __________________________________________________________________________    Composition Nos.                                                                        60   61   62   63   64   65   66                                    __________________________________________________________________________    Ingredients                                                                   SiO.sub.2 73.5 72.5 72.7 72.9 79.3 78.8 65.6                                  Al.sub.2 O.sub.3                                                                        19   18.9 19   19   13.5 13.4 25.9                                  ZnO       1.7  0.6  0.3  0.1  1.3  1.2  0.5                                   Li.sub.2 O                                                                              2.25 4.4  4.4  4.4  2.4  3.2  4.6                                   TiO.sub.2 1.45 1.45 1.45 1.45 1.45 1.45 1.45                                  ZrO.sub.2 1.6  1.6  1.6  1.6  1.6  1.6  1.6                                   Sb.sub.2 O.sub.3                                                                        0.3  0.3  0.3  0.3  0.3  0.3  0.3                                   Na.sub.2 O                                                                              0.1  0.1  0.1  0.1  0.1  0.1  0.1                                   __________________________________________________________________________

While glass-ceramics having high stability when exposed to temperaturesas high as 1,900°F over extended periods of time are made from the SiO₂-Al₂ O₃ -Li₂ O compositions of the first embodiment of the presentinvention, and while glass-ceramics having greater strengths areproduced by the SiO₂ -Al₂ O₃ -Li₂ O-ZnO compositions of the secondembodiment of the invention, many of the latter compositions do notproduce glass-ceramics which are thermally stable at the 1,900°Ftemperatures. However, a small group of compositions containing ZnO andcoming within the second embodiment of the invention, do have therequisite thermal stability at 1,900°F over a long period of time. Suchcompositions can be defined as having:

A. 1. a SiO₂ /Al₂ O₃ molar ratio of 5-8

2. a Li₂ O/Al₂ O₃ molar ratio of 0.55-0.75

3. a ZnO/Li₂ O molar ratio of 0.04-0.5, and

4. a ZnO + Li₂ O/Al₂ O₃ molar ratio: ≦1, or

B. 1. a SiO₂ /Al₂ O₃ molar ratio of 4 to less than 5

2. a Li₂ O/Al₂ O₃ molar ratio of 0.55-0.75

3. a ZnO/Li₂ O molar ratio of 0.06-0.5, and

4. a ZnO + Li₂ O/Al₂ O₃ molar ratio ≦1.

Compositions coming within these critical ratios and having good thermalstability at 1,900°F are Nos.: 39, 48, 49, 51, 53-56.

A matrix for a regenerative heat exchanger as disclosed in theaforementioned U.S. Pat. application Ser. No. 30,859 was made utilizingcomposition 37 of the present invention. Such a matrix, broadlyspeaking, comprises integrally fused tubes forming a series of smoothlongitudinal parallel passageways therethrough wherein the wallsdefining the passageways have (1) essentially zero porosity, (2) consistessentially of an inorganic crystallinne oxide material, (3) a maximumdiameter of 0.1 inch, (4) a maximum thickness of about 0.03 to 0.002inch through portions of the walls common to adjacent fused tubes, withthe ratio of the diameter to such thickness being at least 3. The matrixhas an open frontal area of at least 60% and the walls of the matrixhave a thermal conductivity of less than 0.01 cal/cm/sec/cm² /°C. at400°C.

Glass tubing made from Composition 37 above and having an inner diameterof .030 inches and a wall thickness of .0015 inches was cut into lengths31/2 inches long and each end was sealed, trapping air therein.

The sealed tubing was tightly packed parallel to the length of a moldlines with a layer of heat-resistant alumina-silica paper. The tubeswithin the mold were in a closely-packed parallel relationship with eachother. The assembly was then heated in a kiln on the followingschedule:Temperature Time orRate______________________________________Ambient to 1250°F 300°F/hr.Hold at 1250°F 8 hrs.1250°F to 2100°F 100°F/hr. Hold at 2100°F 2hrs.2100°F to Room Temperature300°F/hr.______________________________________

After this heat treatment, the resulting glass- ceramic matrix wasremoved from the mold and the ends of the tubes opened by grinding. Theforegoing heat treatment thermally in situ crystallized the glass andproduced a glass-ceramic matrix. The heat treatment had fusion bondedthe walls of each tube to those of adjacent tubes and reformed each tubeto substantially hexagonal shape. The matrix was suitable as a heatregenerator for a gas turbine engine, able to withstand the operatingtemperature of at least 1,500°F for an extended period of time withoutany deleterious thermal instability occurring.

The above glass tubing of Composition 37 is also suitable for making theglass-ceramic recuperatore matrix disclosed in U.S. Pat. applicationSer. No. 30,859 and in the U.S. Pat. application Ser. No. 146,665 filedMay 25, 1971, as a divisional and continuation-in-part thereof. There, aplurality of layers of tubes are superimposed one above the other insuccessive parallel planes, the tubes within each plane beingessentially parallel to each other and transverse to tubes in adjacentlayers. When subjected to the same heat schedule described above withrespect to the regenerator matrix, each tube becomes integrally fused toeach adjacent parallel tube and each adjacent transverse tube. When, aspreferred, the tubes are essentially fully expanded, each tube wall is acommon wall with each tube adjacent thereto, including those in the sameplane and those in adjacent parallel planes.

Moreover, when fully expanded, the passageways are essentially in theshape of a parallelogram, usually a square or a rectangle. After theheat treatment and the subsequent opening of the sealed ends, such as bygrinding, an integral glass-ceramic recuperator matrix is obtained.

Using glass tubing formed from the compositions of the presentinvention, such as Composition 37, a core assembly for a thermal reactorfor receiving the flow of hot gas effluents from a combustion engine,such as that disclosed in the aforementioned Zmudz et al U.S. Pat.application filed Mar. 23, 1971 can be made.

Such glass tubing is tightly packed together about a tubular member inparallel relationship, with each end of the tubing being sealed. Theouter periphery of the assembly is then constrained from outwardmovement in a direction perpendicular to the longitudinal passagewaysformed by the tubing and the tubular member. The assembly is thensubjected to a heat schedule such that the tubing becomes softened andthe air entrapped therein expands and urges the tubes into tight contactwith adjacent tubes. A glass-ceramic matrix comprising a plurality ofexpanded tubes fusion-bonded together and to the tubular member is thusformed. The sealed ends of the tubing are opened and the matrix can thenbe utilized in the manner disclosed in the aforesaid Zmuda et alapplication to make the thermal reactor for a combustion engine.

What is claimed is:
 1. A thermally crystallizable glass capable of beingthermally in situ crystallized to a glass-ceramic which is thermallystable at temperatures of about 1,500°F for at least 1000 hours andmaintains its modulus of rupture of at least about 10,000 psi at suchtemperatures for said times, said glass consisting essentially of thefollowing ingredients:

    Ingredients       Weight Percent                                              ______________________________________                                        SiO.sub.2         55 - 80                                                     Al.sub.2 O.sub.3  12 - 27                                                     Li.sub.2 O        3.2 - 7.6                                                   Nucleating Agent  3 - 9                                                       ______________________________________                                    

wherein said nucleating agent is a mixture of TiO₂ + ZrO₂, wherein theZrO₂ does not exceed 3 weight percent, but is at least 0.5 weightpercent, wherein the SiO₂ /Al₂ O₃ molar ratio is from 4 to 10 andwherein a. when the SiO₂ /Al₂ O₃ molar ratio is from 4 to less than 5,the Li₂ O/Al₂ O₃ molar ratio is from 0.75 to 0.97; and b. when the SiO₂/Al₂ O₃ molar ratio is from 5 to 7.5, the Li₂ O/Al₂ O₃ molar ratio isfrom 0.65 to 0.97; and c. when the SiO₂ /Al₂ O₃ molar ratio is more than7.5 and less than 9, the Li₂ O/Al₂ O₃ molar ratio is from 0.8 to 0.97;and d. when the SiO₂ /Al₂ O₃ molar ratio is more than 9 and up to 10,the Li₂ O/Al₂ O₃ molar ratio is from 0.87 to 0.97.
 2. The thermallycrystallizable glass as defined in claim 1, wherein at least 98% byweight of said glass consists of said SiO₂, Al₂ O₃, Li₂ O and nucleatingagent.
 3. A thermally crystallizable glass as defined in claim 1,wherein said glass contains in weight percent no more than 0.5 K₂ O, or0.5 Na₂ O, or 0.5 of a mixture of (Na₂ O+K₂ O).
 4. A thermallycrystallizable glass as defined in claim 1, wherein said glass containsin weight percent no more than 0.25 K₂ O, or 0.25 Na₂ O, or 0.25 of amixture of (Na₂ O+K₂ O).
 5. The thermally crystallizable glass asdefined in claim 1, wherein said glass consists of said SiO₂, Al₂ O₃,Li₂ O and nucleating agent.
 6. The thermally crystallizable glass asdefined in claim 1, wherein ZrO₂ is present in an amount of 0.5 to 2.8weight percent of said composition.
 7. A glass-ceramic article thermallystable at temperatures of about 1,500°F for at least 1,000 hours andretaining its modulus of rupture of at least about 10,000 psi whensubjected to said temperatures for said times, said article being formedby thermally in situ crystallizing a glass consisting essentially of thefollowing ingredients:

    Ingredients       Weight Percent                                              ______________________________________                                        SiO.sub.2         55 - 80                                                     Al.sub.2 O.sub.3  12 - 27                                                     Li.sub.2 O        3.2 - 7.6                                                   Nucleating Agent  3 - 9                                                       ______________________________________                                    

wherein said nucleating agent is a mixture of TiO₂ + ZrO₂, wherein theZrO₂ does not exceed 3 weight percent, but is at least 0.5 weightpercent, wherein the SiO₂ /Al₂ O₃ molar ratio is from 4 to 10, andwherein a. when the SiO₂ /Al₂ O₃ molar ratio is from 4 to less than 5,the Li₂ O/Al₂ O₃ molar ratio is from 0.75 to 0.97; and b. when the SiO₂/Al₂ O₃ molar ratio is from 5 to 7.5, the Li₂ O/Al₂ O₃ molar ratio isfrom 0.65 to 0.97; and c. when the SiO₂ /Al₂ O₃ molar ratio is more than7.5 and less than 9, the Li₂ O/Al₂ O₃ molar ratio is from 0.8 to 0.97;and d. when the SiO₂ /Al₂ O₃ molar ratio is more than 9 and up to 10,the Li₂ O/Al₂ O₃ molar ratio is from 0.87 to 0.97,said glass-ceramicarticle having a coefficient of linear thermal expansion of -12 to +12 ×10.sup.⁻⁷ (0°-700°C).
 8. The glass-ceramic article as defined in claim7, wherein at least 98% by weight of said glass-ceramic consists of saidSiO₂, Al₂ O₃, Li₂ O and nucleating agent.
 9. A glass-ceramic article asdefined in claim 7, wherein said glass-ceramic contains in weightpercent no more than 0.5 K₂ O, or 0.5 Na₂ O, or 0.5 of a mixture of (Na₂O+K₂ O).
 10. A glass-ceramic article as defined in claim 7, which isthermally stable at temperatures of about 1,900°F for at least 1000hours, wherein said glass-ceramic contains in weight percent no morethan 0.25 K₂ O, or 0.25 Na₂ O, or 0.25 of a mixture of (Na₂ O+K₂ O). 11.The glass-ceramic article as defined in claim 7, wherein saidglass-ceramic consists of said SiO₂, Al₂ O₃, Li₂ O and nucleating agent.12. The glass-ceramic article as defined in claim 7, wherein ZrO₂ ispresent in an amount of 0.5 to 2.8 weight percent of said composition.13. The glass-ceramic article as defined in claim 7, wherein saidglass-ceramic has a coefficient of linear thermal expansion of -3 to+3×10.sup.⁻⁷ (0°-700°C.).
 14. The glass-ceramic article as defined inclaim 7, wherein said glass-ceramic has a coefficient of linear thermalexpansion of -0.5 to +0.5×10.sup.⁻⁷ (0°-700°C.).
 15. A thermallycrystallizable glass capable of being thermally in situ crystallized toa glass-ceramic which is thermally stable at temperatures of about1,500°F for at least 1,000 hours and maintains its modulus of rupture ofat least 10,000 psi at such temperatures for said times, said glassconsisting essentially of the following ingredients:

    Ingredients       Weight Percent                                              ______________________________________                                        SiO.sub.2          63 - 78.5                                                  Al.sub.2 O.sub.3  13.8 - 23                                                   Li.sub.2 O        3.5 - 6.5                                                   Nucleating Agent  3 - 9                                                       ______________________________________                                    

wherein said nucleating agent is a mixture of TiO₂ plus ZrO₂, whereinthe ZrO₂ does not exceed 3 weight percent but is at least 0.5 weightpercent, and wherein the SiO₂ /Al₂ O₃ molar ratio is from 5 to less than9, and wherein a. when the SiO₂ /Al₂ O₃ molar ratio is from 5 to lessthan 7.5, the Li₂ O/Al₂ O₃ molar ratio is from 0.75 to 0.97; and b. whenthe SiO₂ /Al₂ O₃ molar ratio is from 7.5 to less than 9, the Li₂ O/Al₂O₃ molar ratio is from 0.87 to 0.97.
 16. A glass-ceramic articlethermally stable at temperatures of about 1,900°F for at least 1,000hours and retaining its modulus of rupture of at least about 10,000 psiwhen subjected to said temperatures for said times, said article beingformed by thermally in situ crystallizing a glass consisting essentiallyof the following ingredients:

    Ingredients       Weight Percent                                              ______________________________________                                        SiO.sub.2          63 - 78.5                                                  Al.sub.2 O.sub.3  13.8 - 23                                                   Li.sub.2 O        3.5 - 6.5                                                   Nucleating Agent  3 - 9                                                       ______________________________________                                    

wherein said nucleating agent is a mixture of TiO₂ plus ZrO₂, whereinthe ZrO₂ does not exceed 3 weight percent but is at least 0.5 weightpercent, and wherein the SiO₂ /Al₂ O₃ molar ratio is from 5 to less than9, and wherein a. when the SiO₂ /Al₂ O₃ molar ratio is from 5 to lessthan 7.5, the Li₂ O/Al₂ O₃ molar ratio is from 0.75 to 0.97; and b. whenthe SiO₂ /Al₂ O₃ molar ratio is from 7.5 to less than 9, the Li₂ O/Al₂O₃ molar ratio is from 0.87 to 0.97,said glass-ceramic having acoefficient of linear thermal expansion of -12 to +12 × 10.sup.⁻⁷(0°-700°C).
 17. A glass-ceramic article thermally stable at temperaturesof about 1,900°F for at least 1000 hours and retaining its modulus ofrupture of at least about 10,000 psi when subjected to said temperaturesfor said times, said article being formed by thermally in situcrystallizing a glass consisting essentially of the followingingredients:

    Ingredients       Weight Percent                                              ______________________________________                                        SiO.sub.2         62.8 - 80                                                   Al.sub.2 O.sub.3  12.7 - 26.7                                                 Li.sub.2 O        3.2 - 7.6                                                   Nucleating Agent  3 - 9                                                       ______________________________________                                    

wherein said nucleating agent is a mixture of TiO₂ + ZrO₂, wherein theZrO₂ does not exceed 3 weight percent, but is at least 0.5 weightpercent, wherein the SiO₂ /Al₂ O₃ molar ratio is from 4 to 10, andwherein a. when the SiO₂ /Al₂ O₃ molar ratio is from 4 to less than 5,the Li₂ O/Al₂ O₃ molar ratio is from 0.75 to 0.97; and b. when the SiO₂/Al₂ O₃ molar ratio is from 5 to 7.5, the Li₂ O/Al₂ O₃ molar ratio isfrom 0.65 to 0.97; and c. when the SiO₂ /Al₂ O₃ molar ratio is more than7.5 and less than 9, the Li₂ O/Al₂ O₃ molar ratio is from 0.8 to 0.97;and d. when the SiO₂ /Al₂ O₃ molar ratio is more than 9 and up to 10,the Li₂ O/Al₂ O₃ molar ratio is from 0.87 to 0.97,said glass-ceramicarticle having a coefficient of linear thermal expansion of -12 to +12 ×10.sup.⁻⁷ (0°-700°C).
 18. A thermally crystallizable glass capable ofbeing thermally in situ crystallized to a glass-ceramic which isthermally stable at temperatures of about 1,500°F for at least 1000hours, said glass consisting essentially of the following ingredients:

    Ingredients       Weight Percent                                              ______________________________________                                        SiO.sub.2          52 - 78.4                                                  Al.sub.2 O.sub.3  12.8 - 27.5                                                 Li.sub.2 O        2.4 - 7                                                     ZnO               0.25 - 6.7                                                  Nucleating Agent  3 -  9                                                      ______________________________________                                    

wherein said nucleating agent is selected from the group consisting ofZrO₂ and a mixture of TiO₂ and ZrO₂ and wherein the ZrO₂ is present inan amount not in excess of 3% by weight, but is at least 0.5 weightpercent, wherein the SiO₂ /Al₂ O₃ molar ratio is from 4 to 9 and theZnO + Li₂ O/Al₂ O₃ molar ratio is ≦ 1, and a. when the SiO₂ /Al₂ O₃molar ratio is from 4 to less than 5, the Li₂ O/A₂ O₃ molar ratio isfrom 0.55 to 0.91, and the ZnO/Li₂ O molar ratio is from 0.06 to 0.5;and b. when the SiO₂ /Al₂ O₃ molar ratio is from 5 to 8, the Li₂ O/Al₂O₃ molar ratio is from 0.55 to 0.93, and the ZnO/Li₂ O molar ratio isfrom 0.04 to 0.5; and c. when the SiO₂ /Al₂ O₃ molar ratio is from morethan 8 up to 9, the Li₂ O/Al₂ O₃ molar ratio is from 0.8 to 0.93, andthe ZnO/Li₂ O molar ratio is from 0.04 to 0.5; and d. the glass containsin weight percent no more than 0.2 K₂ O, or 0.2 Na₂ O, or 0.2 (Na₂ O +K₂ O).
 19. The thermally crystallizable glass as defined in claim 18wherein at least 98% by weight of said glass consists of said SiO₂, Al₂O₃, Li₂ O, ZnO and nucleating agent.
 20. The thermally crystallizableglass as defined in claim 18 wherein the nucleating agent is a mixtureof ZrO₂ plus TiO₂ and said ZrO₂ is present in an amount of 0.5 to 2.8weight percent of said composition.
 21. A glass-ceramic articledimensionally stable at temperatures up to about 1,500°F for at least1,000 hours and retaining its modulus of rupture of at least about 13,00psi when subjected to said temperatures for said times, said articlebeing formed by thermally in situ crystallizing a glass consistingessentially of the following ingredients

    Ingredients       Weight Percent                                              ______________________________________                                        SiO.sub.2         52 - 78.4                                                   Al.sub.2 O.sub.3  12.8 - 27.5                                                 Li.sub.2 O        2.4 - 7                                                     ZnO               0.25 - 6.7                                                  Nucleating Agent  3 - 9                                                       ______________________________________                                    

wherein said nucleating agent is selected from the group consisting ofZrO₂ and a mixture of TiO₂ and ZrO₂ and wherein the ZrO₂ is present inan amount not in excess of 3% by weight, but is at least 0.5 weightpercent, wherein the ZnO + Li₂ O/Al₂ O₃ molar ratio is ≦ 1, and whereinthe SiO₂ /Al₂ O₃ molar ratio is from 4 to 9 and a. when the SiO₂ /Al₂ O₃molar ratio is from 4 to less than 5, the Li₂ O/Al₂ O₃ molar ratio isfrom 0.55 to 0.91, and the ZnO/Li₂ O molar ratio is from 0.06 to 0.5;and b. when the SiO₂ /Al₂ O₃ molar ratio is from 5 to 8, the Li₂ O/Al₂O₃ molar ratio is from 0.55 to 0.93, and the ZnO/Li₂ O molar ratio isfrom 0.04 to 0.5; and c. when the SiO₂ /Al₂ O₃ molar ratio is from morethan 8; up to 9, the Li₂ O/Al₂ O₃ molar ratio is from 0.8 to 0.93 andthe ZnO/Li₂ O molar ratio is from 0.04 to 0.5; and d. the glass andresulting glass-ceramic contains in weight percent no more than 0.2 K₂O, or 0.2 Na₂ O, or 0.2 (Na₂ O + K₂ O),said glass-ceramic having acoefficient of linear thermal expansion of -12 to +12 × 10⁻ ⁷(0°-700°C).
 22. The glass-ceramic article as defined in claim 21 whereinat least 98% by weight of said glass-ceramic consists of said SiO₂, Al₂O₃, Li₂ O, ZnO and nucleating agent.
 23. The glass-ceramic article asdefined in claim 21 wherein the nucleating agent is a mixture of ZrO₂plus TiO₂ and said ZrO₂ is present in an amount of 0.5 to 2.8 weightpercent of said composition.
 24. The glass-ceramic article as defined inclaim 21 wherein said glass-ceramic has a coefficient of linear thermalexpansion of -3 to +3 ×10⁻ ⁷ (0°-700° C.).
 25. A thermallycrystallizable glass capable of being thermally in situ crystallized toa glass-ceramic which is thermally stable at temperatures of about1,500°F for at least 1,000 hours, said glass consisting essentially ofthe following ingredients:

    Ingredients       Weight Percent                                              ______________________________________                                        SiO.sub.2         58.2 - 78.4                                                 Al.sub.2 O.sub.3  13.7 - 26.5                                                 Li.sub.2 O        3.2 - 6.9                                                   ZnO               0.3 - 6.5                                                   Nucleating Agent  3 - 9                                                       ______________________________________                                    

wherein said nucleating agent is selected from the group consisting ofZrO₂ and a mixture of TiO₂ and ZrO₂ and wherein the ZrO₂ is present inan amount not in excess of 3% by weight, but is at least 0.5 weightpercent, wherein the SiO₂ /Al₂ O₃ molar ratio is from 4 to 9 and theZnO + Li₂ O/Al₂ O₃ molar ratio is ≦ 1, and a. when the SiO₂ /Al₂ O₃molar ratio is from 4 to less than 5, the Li₂ O/Al₂ O₃ molar ratio isfrom 0.55 to 0.91, and the ZnO/Li₂ O molar ratio is from 0.06 to 0.5; b.when the SiO₂ /Al₂ O₃ molar ratio is from 5 to 8, the Li₂ O/Al₂ O₃ molarratio is from 0.55 to 0.93, and the ZnO/Li₂ O molar ratio from 0.04 to0.5; and c. when the SiO₂ O/Al₂ O₃ molar ratio is from more than 8 up to9, the Li₂ O/Al₂ O₃ molar ratio is from 0.8 to 0.93 and the ZnO/Li₂ Omolar ratio is from 0.04 to 0.5; and d. the glass contains in weightpercent no more than 0.2 K₂ O, or 0.2 Na₂ O, or 0.2 (Na₂ O + K₂ O). 26.The thermally crystallizable glass as defined in claim 25 wherein atleast 98% by weight of said glass consists of said SiO₂, Al₂ O₃, ZnO andnucleating agent.
 27. A thermally crystallizable glass according toclaim 18 capable of being thermally in situ crystallized to aglass-ceramic which is thermally stable at temperatures of about 1,900°Ffor at least 1,000 hours, said glass being selected from the groupconsisting ofA. glass havinga. a SiO₂ /Al₂ O₃ molar ratio of 5-8 b. aLi₂ O/Al₂ O₃ molar ratio of 0.55 to 0.75, and c. a ZnO/Li₂ O molar ratioof 0.04 - 0.5 B. a glass havinga. a SiO₂ /Al₂ O₃ molar ratio of 4 toless than 5 b. a Li₂ O/Al₂ O₃ molar ratio of 0.55 - 0.75, and c. aZnO/Li₂ O molar ratio of 0.06 - 0.5.
 28. The glass-ceramic article ofclaim 16 wherein said ceramic has a dimensional stability of less than250 parts per million after being subjected to a temperature of 1,900°Ffor at least 1,000 hours.
 29. The glass-ceramic article of claim 21wherein said ceramic has a dimensional stability of less than 250 partsper million after being subjected to a temperature of 1,500°F for atleast 1,000 hours.
 30. The glass-ceramic article of claim 29 whereinsaid dimensional stability is retained after said article is subjectedto said 1,500°F temperature for 2,000 hours.
 31. A glass-ceramicaccording to claim 21 dimensionally stable at temperatures of about1,900°F for at least 1,000 hours and maintaining its modulus of ruptureof at least 13,000 psi when subjected to such temperatures for saidtimes, said article being formed by thermally in situ crystallizing aglass selected from the group consisting ofA. a glass havinga. SiO₂ /Al₂O₃ molar ratio of 5-8 b. a Li₂ O/Al₂ O₃ molar ratio of 0.55 to 0.75 c. aZnO/Li₂ O molar ratio of 0.04 - 0.5, and d. a ZnO+Li₂ O/Al₂ O₃ molarratio ≦ 1; and B. a glass havinga. a SiO₂ /Al₂ O₃ molar ratio of 4 toless than 5 b. a Li₂ O/Al₂ O₃ molar ratio of 0.55 - 0.75 c. a ZnO/Li₂ Omolar ratio of 0.06 - 0.5, and d. a ZnO+Li₂ O/Al₂ O₃ molar ratio ≦ 1saidglass-ceramic having a coefficient of linear thermal expansion of -12 to+12 ×10⁻ ⁷ (0°-700°C.).
 32. A thermally crystallizable glass capable ofbeing thermally in situ crystallized to a glass-ceramic which isthermally stable at temperatures of about 1,500°F for at least 1,000hours and maintains its modulus of repture of at least about 10,000 psiat such temperatures for said times, said glass consisting of thefollowing ingredients, exclusive of fining agents:

    Ingredients       Weight Percent                                              ______________________________________                                        SiO.sub.2         55 - 80                                                     Al.sub.2 O.sub.3  12 - 27                                                     Li.sub.2 O        3.2 - 7.6                                                   Nucleating Agent  3 - 9                                                       ______________________________________                                         wherein said nucleating agent is a mixture of TiO.sub.2 + ZrO.sub.2,     wherein the ZrO.sub.2 does not exceed 3 weight percent, but is at least     0.5 weight percent, wherein the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio is     from 4 to 10 and wherein

a. when the SiO₂ /Al₂ O₃ molar ratio is from 4 to less than 5, the Li₂O/Al₂ O₃ molar ratio is from 0.75 to 0.97; and b. when the SiO₂ /Al₂ O₃molar ratio is from 5 to 7.5, the Li₂ O/Al₂ O₃ molar ratio is from 0.65to 0.97; and c. when the SiO₂ /Al₂ O₃ molar ratio is more than 7.5 andless than 9, the Li₂ O/Al₂ O₃ molar ratio is from 0.8 to 0.97; and d.when the SiO₂ /Al₂ O₃ molar ratio is more than 9 and up to 10, to Li₂O/Al₂ O₃ molar ratio is from 0.87 to 0.97.
 33. A glass-ceramic articlethermally stable at temperatures of about 1,500°F for at least 1,000hours and retaining its modulus of rupture of at least about 10,000 psiwhen subjected to said temperatures for said times, said article beingformed by thermally in situ crystallizing a glass consisting of thefollowing ingredients, exclusive of fining agents:

    Ingredients       Weight Percent                                              ______________________________________                                        SiO.sub.2         55 - 80                                                     Al.sub.2 O.sub.3  15 - 27                                                     Li.sub.2 O        3.2 - 7.6                                                   Nucleating Agent  3 - 9                                                       ______________________________________                                    

wherein said nucleating agent is a mixture of TiO₂ + ZrO₂, wherein theZrO₂ does not exceed 3 weight percent, but is at least 0.5 weightpercent, wherein the SiO₂ /Al₂ O₃ molar ratio is from 4 to 10, andwherein a. when the SiO₂ /Al₂ O₃ molar ratio is from 4 to less than 5,the Li₂ O/Al₂ O₃ molar ratio is from 0.75 to 0.97; and b. when the SiO₂/Al₂ O₃ molar ratio is from 5 to 7.5 the Li₂ O/Al₂ O₃ molar ratio isfrom 0.67 to 97; and c. when the SiO₂ /Al₂ O₃ molar ratio is more than7.5 and less than 9, the Li₂ O/Al₂ O₃ molar ratio is from 0.8 to 0.97;and d. when the SiO₂ /Al₂ O₃ molar ratio is more than 9 and up to 10,the Li₂ O/Al₂ O₃ molar ratio is from 0.87 to 0.97.said glass-ceramicarticle having a coefficient of linear thermal expansion of -12 to +12 ×10⁻ ⁷ (0°-700°C).
 34. A thermally crystallizable glass capable of beingthermally in situ crystallized to a glass-ceramic which is thermallystable at temperatures of about 1,500°F for at least 1,000 hours, saidglass consisting of the following ingredients, exclusive of finingagents:

    Ingredients       Weight Percent                                              ______________________________________                                        SiO.sub.2         52 - 78.4                                                   Al.sub.2 O.sub.3  12.8 - 27.5                                                 Li.sub.2 O        2.4 - 7                                                     ZnO               0.25 - 6.7                                                  Nucleating Agent  3 - 9                                                       ______________________________________                                    

wherein said nucleating agent is selected from the group consisting ofZrO₂ and a mixture of TiO₂ and ZrO₂ and wherein the ZrO₂ is present inan amount not in excess of 3% by weight, but is at least 0.5 weightpercent, wherein the SiO₂ /Al₂ O₃ molar ratio is from 4 to 9 and theZnO + Li₂ O/Al₂ O₃ molar ratio is 1, and a. when the SiO₂ /Al₂ O₃ molarratio is from 4 to less than 5, the Li₂ O/Al₂ O₃ molar ratio is from0.55 to 0.91, and the ZnO/Li₂ O molar ratio is from 0.06 to 0.5; and b.when the SiO₂ /Al₂ O₃ molar ratio is from 5 to 8, the Li₂ O/Al₂ O₃ molarratio is from 0.55 to 0.93, and the ZnO/Li₂ O molar ratio is from 0.04to 0.5; and c. when the SiO₂ /Al₂ O₃ molar ratio is from more than 8 upto 9, the Li₂ O/Al₂ O₃ molar ratio is from 0.8 to 0.93, and the ZnO/Li₂O molar ratio is from 0.04 to 0.5; and d. the glass contains in weightpercent no more than 0.2 K₂ O, or 0.2 Na₂ O, or 0.2 (Na₂ O + K₂ O). 35.A glass-ceramic article dimensionally stable at temperatures up to about1,500°F for at least 1,000 hours and retaining its modulus of rupture ofat least about 13,000 psi when subjected to said temperatures for saidtimes, said article being formed by thermally in situ crystallizing aglass consisting of the following ingredients, exclusive of finingagents:

    Ingredients       Weight Percent                                              ______________________________________                                        SiO.sub.2         52 - 78.4                                                   Al.sub.2 O.sub.3  12.8 - 27.5                                                 Li.sub.2 O        2.4 - 7                                                     ZnO               0.25 - 6.7                                                  Nucleating Agent  3 - 9                                                       ______________________________________                                    

wherein said nucleating agent is selected from the group consisting ofZrO₂ and a mixture of TiO₂ and ZrO₂ and wherein the ZrO₂ is present inan amount not in excess of 3% by weight, but is at least 0.5 weightpercent, wherein the ZnO + Li₂ O/Al₂ O₃ molar ratio is 1, and whereinthe SiO₂ /Al₂ O₃ molar ratio is from 4 to 9 and a. when the SiO₂ /Al₂ O₃molar ratio is from 4 to less than 5, the Li₂ O/Al₂ O₃ molar ratio isfrom 0.55 to 0.91, and the ZnO/Li₂ O molar ratio is from 0.06 to 0.5;and b. when the SiO₂ /Al₂ O₃ molar ratio is from 5 to 8, the Li₂ O/Al₂O₃ molar ratio is from 0.55 to 0.93 and the ZnO/Li₂ O molar ratio isfrom 0.04 to 0.5; and c. when the SiO₂ /Al₂ O₃ molar ratio is from morethan 8 up to 9, the Li₂ O/Al₂ O₃ molar ratio is from 0.8 to 0.93 and theZnO/Li₂ O molar ratio is from 0.04 to 0.5; and d. the glass resultingglass-ceramic contains in weight percent no more than 0.2 K₂ O, or 0.2Na₂ O or 0.2 (Na₂ O + K₂ O).said glass-ceramic having a coefficient oflinear thermal expansion of -12 to +12 × 10⁻ ⁷ (0°-700°C).